BILATERALITY OF THE PIGEON's EGG 271 



R. Lillie for his constant interest, advice and criticism. I wish 

 to thank Prof. C. L. Bristol, Dr. J. T. Patterson and the members 

 of this Department for their suggestions and cooperation. Dr. 

 R. R. Bensley has shown me many courtesies in the course of the 

 work; his artist, Mr. A. B. Streedain copied figs. 42 and 43 from 

 the original drawings. 



II. MATERIAL AND METHODS 



Harper ('04), Blount ('09), and Patterson ('09) have discussed 

 in detail the man}- advantages the pigeon's egg has for studies 

 on the early stages of development in the bird's egg. The 

 technique employed .is new for these eggs and will be discussed 

 briefly. 



At the outset the necessity of studying the living material must 

 be emphasized. No conclusions can be based on the form-rela- 

 tions of the younger oocytes or of the blastodiscs that are derived 

 solely from preserved material. Certain distortions are unavoid- 

 able, even with the most careful treatment, and they must be 

 controlled by the study and measurement of the living cells. The 

 greatest distortions are introduced in cutting thin paraffin sec- 

 tions, so these have been used only to add structural details and 

 for the illustrations. In the latter case they were used because 

 it is easy to photograph them; they are to be considered merely 

 as corroborative evidence as to the conditions observed in fresh 

 or creasote preparations and free hand sections. Entire ovaries 

 cleared in creasote, dissected and studied under the binocular as 

 solid objects afford an easier and more accurate means of inter- 

 preting form-relations than could possibly be obtained from the 

 study of sections, and this method, when properly controlled, is 

 to be highly recommended. In this three-dimensional study of 

 the oocytes the following technique was used: Fix the entire 

 ovary for not more than six hours in a mixture consisting of: 



Saturated HgCl-i in normal salt solution 94 cc. 



(llacial acetic acid 6 cc. 



Neutral forniol (comm. formaldehyde solution neutralized with 



MgCOs) ' '. etolOcc. 



